Various types of batteries have been developed heretofore, and in every battery, a packaging material is an essential member for sealing battery elements such as an electrode and an electrolyte. Metallic packaging materials have been often used heretofore as battery packagings.
On the other hand, in recent years, batteries have been required to be diversified in shape and to be thinned and lightened with improvement of performance of electric cars, hybrid electric cars, personal computers, cameras, mobile phones and so on. However, metallic battery packaging materials that have been often used heretofore have the disadvantage that it is difficult to keep up with diversification of shapes, and there is a limit to weight reduction.
Thus, in recent years, there has been proposed a film-shaped laminate with a base material, a metal layer and a sealant layer laminated in this order has been proposed as a battery packaging material which is easily processed into diversified shapes and is capable of achieving thickness reduction and weight reduction. However, such a film-shaped packaging material is thinner as compared to a metallic packaging material, and has the disadvantage that pinholes and cracks are easily generated during molding. If pinholes and cracks are generated in a battery packaging material, an electrolytic solution may permeate to a metal layer to form a metal precipitate, resulting in occurrence of a short-circuit, and therefore it is absolutely necessary that a film-shaped battery packaging material have a property that makes it hard to generate pinholes during molding, i.e. excellent moldability.
Various studies have been conducted heretofore with attention paid to an adhesive layer for bonding a metal layer in order to improve the moldability of a film-shaped battery packaging material. For example, Patent Document 1 discloses that in a laminated packaging material which includes an inner layer including a resin film; a first adhesive agent layer; a metal layer; a second adhesive agent layer; and an outer layer including a resin film, at least one of the first adhesive agent layer and the second adhesive agent layer is formed of an adhesive composition containing a resin having an active hydrogen group on the side chain, a polyfunctional isocyanate and a polyfunctional amine compound, a packaging material having high reliability in deeper molding is obtained.
As represented by Patent Document 1, many studies have been conducted heretofore on techniques for improving moldability with attention paid to blended components of an adhesive layer for bonding a metal layer and other layer in a battery packaging material including a film-shaped laminate, but there have been few reports techniques for improving moldability with attention paid to the properties of a metal layer.
It is known that generally, a metal material having a low yield strength and a large tensile strength is not only easily deformed, but also hardly creased in deep drawing, and thus has excellent processability (see Non-Patent Document 1), and heretofore, a metal raw material having a low yield strength has been generally employed as a metal layer in a battery packaging material including a film-shaped laminate.
Further, a battery packaging material is required to have high electrolytic solution resistance, and for example, by using a polyester film as a base material layer of the battery packaging material, electrolytic solution resistance can be further improved. However, a polyester film is inferior in moldability to a polyamide film etc., and the use of a polyester film as a base material layer causes the problem that pinholes are easily generated during molding. Therefore, when a polyester film is used as a base material layer for improving electrolytic solution resistance, it is particularly difficult to improve the moldability of a battery packaging material.
For improving the moldability of a battery packaging material, a nylon film is widely used as a base material (see, for example, Patent Document 2). However, there is the problem that if an electrolytic solution is deposited on a base material surface in production of a battery using a battery packaging material including a nylon film as a base material, the base material surface is whitened or melted.
On the other hand, as described above, a polyester film (e.g. polyethylene terephthalate (PET) film) excellent in electrolytic solution resistance may be used as a base material for improving the electrolytic solution resistance of a base material surface. However, a polyester film is poor in moldability, and therefore has the problem that pinholes are easily generated during molding.